Area Forecast Discussion
Issued by NWS Wilmington, NC

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FXUS62 KILM 221416

Area Forecast Discussion
National Weather Service Wilmington NC
1015 AM EDT Sat Jul 22 2017

Hot and humid conditions will peak today and tomorrow before
beginning to moderate with more typical summertime heat expected
next week. A cold front, approaching from the north, should push
into the area mid week and then remain in close proximity through
late week. The risk for thunderstorms with heavy rain will return
next week.


As of 1015 AM Saturday...Primary forecast challenge today is related
to heat and humidity, and subsequent high heat indices. 1200 UTC
soundings support high temperatures from the lower to mid 90s at the
coast to the upper 90s inland. In fact H85 temperatures from the
morning RAOBs range from 19C at the coast to around 20C at KGSO.
Combined with dewpoints in the mid 70s the heat index is expected to
range from around 100 at the immediate coast to 105-109 inland
areas. No changes made to the current Heat Advisory. Otherwise,
subtropical ridging extends from the central plains and MS valley
eastward across the Carolinas and into the western Atlantic. The mid-
level ridging will supers any widespread convective development
with the bulk of any severe weather to be found in the band of
westerlies farther north. However, weak inland troughing could
provide enough lift for a few storms to develop this afternoon and
into the early evening. Thus the highest POPs (albeit on the low-
side) will be found across the far inland areas. Anomalously higher
than normal low temperatures are forecast again tonight with lows in
the mid to upper 70s most areas.


As of 300 AM Saturday...The Forecast Area will be in between
two areas of high pressure. One area of hot high pressure will
retrograde westward as the Bermuda High remains to our E. This
will allow an upper trough to slowly dig across the eastern
CONUS. At the surface, the Piedmont trough will remain
prominent through Mon while a cold front slowly slips down the
east coast, likely lying just to our N Tue morning. This
scenario will allow for still humid, but slightly lower high
temps, especially by Mon. Clouds will be more prevalent Mon as
compared to Sun. The risk for thunderstorms will be increasing
and should be highest Mon and Mon night as precipitable water
values climb to near 2.25 inches. At this time, will show small
POPs Sun with POPs trending higher and perhaps becoming numerous
by Mon/Mon night. High moisture content should allow for very
heavy rainfall amounts where thunderstorms are able to train.

Temps on Sun will still be in the lower to mid 90s which will
produce heat index values at or above 105 degrees across most
areas and so a Heat Advisory may be required that day. On Mon,
highs will be in the lower 90s. Heat index values on Mon are
not expected to exceed 105 degrees, but may still top 100. Lows
will be in the mid and upper 70s although if thunderstorms
become widespread enough a cold pool may drop readings into the
lower 70s.


As of 300 PM Friday...A shortwave trough moving through New
England on Monday night, and a large upper low developing
across southern Quebec will help put a dent in the heat.
Falling upper level heights associated with these systems will
shift the core of the upper ridge (and its associated heat)
farther west into Texas and New Mexico. Don`t expect a cleansing
frontal passage by stretch, (06Z GFS ensemble 850 mb temps
don`t fall below climatology next week) but at least we should
lose the 100+ heat indices for a few days.

Both the GFS and ECMWF show a cold front moving into northern
North Carolina on Tuesday. The GFS then sinks the boundary a
little farther south by Wednesday morning. There are enough GFS
ensemble members showing the front making it into South Carolina
that I`m leaning toward the GFS in this case. This front should
enhance the coverage of t-storms Tuesday night and Wednesday,
and I`ve placed my highest forecast PoPs (50-60 percent) of the
extended period during this period.

With a progressive upper level pattern to our north, surface
high pressure will push off New England and out into the
western Atlantic Thursday and Friday, probably with enough
southerly wind developing over the Carolinas to lift the front
or its remnants back to the north.


As of 12Z...Patchy IFR stratus and MVFR BR will burn off by 12z.
VFR conditions are expected. However, there is potential for
IFR stratus and some fog to redevelop late tonight. The
seabreeze will again develop with SSW winds this afternoon and
early eve gusting to around 20 kt at KCRE and KMYR. No organized
thunderstorms during the period and a remote pop up shower or
thunderstorm during the heating of the day is too low a
probability for inclusion in the TAFs at this time.

Extended Outlook...Possible IFR in patchy stratus/fog near
sunrise Sun. Brief MVFR/IFR possible in showers and thunderstorms
Sun night through Wed.


As of 1015 AM Saturday...Southwest flow around 10 knots currently
prevails per local marine observations. The flow will increase
as the pressure gradient tightens a notch between inland troughing
and high pressure located far off the Southeast U.S. coast. Breezy
southwest flow will persist into tonight, then diminish to 10-15
knots a few hours prior to day break. Choppy seas are expected as
the southwest fetch increases across the waters. Seas of 3 to 5 ft
appear reasonable given the strength of the fetch by this evening.

As of 300 AM Saturday...Small Craft Should Exercise Caution
headlines may be required for at least a portion of the time.

A Piedmont trough on Sun will allow for a tightening pressure
gradient across the waters. As upper trough slowly digs across
the eastern CONUS, a cold front will slip progressively south,
likely lying just N of the waters Tue morning. The approach of
this front will keep the gradient rather tight as well. The wind
direction will be SW through the period. Wind speeds will be as
high as around 20 kt Sun afternoon and eve and again Mon
afternoon. Seas will be 4 to 5 ft through the period.

As of 300 PM Friday...Bermuda High Pressure well offshore will
lose its influence on our weather by the middle of next week as
low pressure in the Canadian maritime provinces pushes a cold
front down the U.S. East Coast. This front is shown by the
latest GFS and ECMWF models to make it down to near Hatteras on
Tuesday. Models diverge slightly on Wednesday as the ECMWF keeps
the front stalled to our north and the GFS brings it down to
the Myrtle Beach vicinity Wednesday morning. Regardless of which
solution is correct, the frisky southwest winds of the weekend
should diminish by Tuesday within the weaker pressure gradient
near the front. Unfortunately thunderstorm potential should also
increase as the front approaches.


SC...Heat Advisory from 11 AM this morning to 7 PM EDT this evening
     for SCZ017-023-024-032-033-039-053>056.
NC...Heat Advisory from 11 AM this morning to 7 PM EDT this evening
     for NCZ087-096-099-105>110.



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